[@realm/react] Export a createAppContext

[kraenhansen]

Problem

Some users want to access data from multiple Atlas clusters in a single app, meaning they'd want to access multiple App contexts from a single React component tree.

Solution

Similarly to the createRealmContext function we're exporting, we could export a createAppContext to help support this pattern of usage.

Alternatives

No response

How important is this improvement for you?

Would be a major improvement

Feature would mainly be used with

Atlas Device Sync